ARM: 8686/1: iwmmxt: Add missing __user annotations to sigframe accessors

preserve_iwmmxt_context() and restore_iwmmxt_context() lack __user
accessors on their arguments pointing to the user signal frame.

There does not be appear to be a bug here, but this omission is
inconsistent with the crunch and vfp sigframe access functions.

This patch adds the annotations, for consistency.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>